The focus of this project is to investigate the mechanisms whereby genetic infomation is transmitted to progeny somatic cells with fidelity: how mutagenesis occurs, and what mechanisms the cell employs to avoid mutation. Using a combination of classical genetic and recombinant DNA techniques, we have contructed a model system to examine the molecular basis of mutagenesis in the yeast, Saccharomyces cerevisiae. Construction of a yeast tester strain has been completed which allows the mutagenesis of a cloned SUP4-o tRNA suppressor gene to be assayed by direct genetic selection and DNA sequence analysis. This tester strain will allow the role in mutagenesis of the three genetically defined repair pathways of yeast to be examined using the same target plasmid. Using this system, the spontaneous mutation rate in the target gene has been determined to be 2.7 x 10-7 events per cell division. Isogenic strains are being constructed which will allow the role of the cloned rad1 gene, required for excision repair in yeast, to be examined in the distribution of mutations which arise folowing ultraviolet irradiation. The SUP4-o system is being developed as a rapid genetic test for the induction of all types of mutation occurring within a eukaryotic gene which will also allow determination of the mutagenic specificities of agents giving positive responses.